Method of producing ferrite bodies



M r 1963 E. c. LEAYCRAFT ETAL 3,

METHOD OF PRODUCING F ERRITE BODIES Filed Nov. 10, 1959 3 Sheets-Sheet 1 FIG.1

I2 FULL SELECT"0 SELECT PULSES 2s FULL SELECT"?! HALF7 24 SELECT "0" HALF 5.2 FVI 5.0

25 100150 250 500 CONTINUOUS FIRING INTERRUPTED FIRING INVENTORS QUENGH TEMP. IN DEGREES GENTIGRADE EDGA R G. LEAYGRAFT GEORGE H. MORRIS,JR.

March 1963 E. c. LEAYCRAFT ETAL 3,

METHOD OF PRODUCING FERRITE BODIES Filed Nov. 10. 1959 3 Sheets-Sheet 5 105- uva I 35 uvi VI \IIVO .1\ 1.40 a is 15 Sec. 1.30

METHOD (FF PRUDUQING FERRITE BGDEES Edgar C. Leaf craft, Woodstock, and George H. Morris,

.lr., Hopewell Junction, N.Y., assignors to international Business Machines Corporation, New York, N.Y., a

corporation of New York Filed Nov. 1t 1959, Ser. No. 852,0%7 2 Claims. (Cl. 252-625) This invention relates to ferrite magnetic materials of the spinel type generally referred to as ferrospinels, and relates particularly to an improved method for processing bodies of such materials so as to provide bodies having improved squareness of the hysteresis characteristic.

Ferrospinel bodies are employed as magnetic memory elements and as pulse transfer elements in computers and other data processing apparatus. When the ferrospinels are employed as memory devices, the squareness of the hysteresis characteristic is of particular importance. The most usual application requiring a maximum of hysteresis squareness is the application involving the use of ferrospinel bodies for coincident current memory devices in which the bodies have a high degree of squareness making possible to switch the magnetic state of the bodies upon the occurance of two simultaneously existing current pulses, one of which alone is of insufiicient intensity to produce magnetic switching. This type of memory device is well known in the art.

Ferrospinel bodies are produced by sintering bodies pressed from mixed powders of ferric oxide and one or more bivalent metal oxides. During the sintering operation, the constituents of the molded bodies arrange themselves to form a spinel type crystal structure. Processes and compositions for producing these ferrospinel structures are well known.

The primary object of the present invention is to improve the squareness characteristic obtained from these spinel type crystal structures and this improvement isbrought about by interrupting the sintering process by cooling the cores and, thereafter, reheating the cores and continuing the sintering process for the completion of the desired sintering time interval.

As previously noted, a square loop memory body desirably exhibits a maximum possible amount of squareness in order that its magnetic state will be substantially undisturbed by a pulse having one half the intensity of a pulse capable of changing the magnetic state of the body. In all such bodies, however, there is some degree of disturbance resulting from half select pulses being applied thereto. The result of this disturbance is to reduce, to some degree, the density of magnetization retained by the body.

The body is capable of retaining one of two opposite states of magnetization, one of these may be considered as being as 1 state and the other may be considered as being a state. Vhen a body is driven from one of these states to the other by means of the application of a magnetic driving force, an output is produced which may be sensed on a suitable sense line and the amplitude of the output may be measured in milli-volts. Thus, it' a body is in the 1 state and it has applied thereto a driving pulse to drive it to a 0 state, there will be roduced What may be termed as a full select or undisturbed l output voltage V On the other hand, if the body was in the 0 state and a full select 0 driving pulse is applied, there will be produced only a very small resulting select output voltage V if a body is in the 1 state and a half select 0 drive pulse is applied, the net result will be to reduce the degree of magnetization remaining in the body. The extent of this reduction will be determined by the square- 3,083,164 Patented Mar. 26, 1963 ness of the hysteresis loop of the body. In view of the fact that a body used as a coincident current device may receive a plurality of half select pulses before receiving a full select pulse, the reduction by each pulse must be kept as small as possible to minimize the diminution of the full select output signal.

Similarly, if the body is in the 0 state and a half select l'drive pulse is applied, the degree of magnetization in the 0 direction will also be reduced. This reduction in magnetization gives rise to an output upon a 9 selection which appears as a noise signal and must therefore be minimized.

As will be hereinafter more fully described, the hysteresis squareness of a body may be defined as the ratio of the output of switching from a disturbed 1 state to the 0 state, expressed as V divided by the output of switching from a disturbed 0 state to the 0 state, expressed as V It is a further object of the invention to improve the .V V ratio of magnetic ferrite cores.

The foregoing and other objects, features and advantages oi the invention will be apparent from the following more particular description of specific embodiments of the invention as illustrated in the accompanying drawings.

In the drawings:

FIGURE 1 shows a hysteresis loop and indicates diagrammatically full and half select pulses for switching a body represented by the loop from one magnetic state to another.

FEGURE 2 is a diagrammatic showing of a sintering cycle in accordance with the invention.

FiGURE 3 is a chart showing the effects on the V V ratio of cooling to various temperatures during the sintering cycle.

FIGURES 4a and 4b are charts showing respective V V ratios of cores produced by interrupted sintering cycles and cores produced by uninterrupting sintering cycles.

As has been previously noted, ferrospinel bodies employed as magnetic memory elements are desirably possessed of a square hysteresis characteristic. In FIGURE 1, there is indicated generally at 10, a hysteresis loop of such a body. The loop is drawn on conventional B and H coordinates. If there is applied to the body a full select 1 driving force on the H axis as indicated by the pulse 12, the body will be driven to a +B state or a l state as indicated by the point 14 on the loop, and, when the driving force is relieved, the residual magnetism in the core will be at a value indicated by the point 16 on the B axis. Similarly, if a full select 0 drive pulse 13 is applied to the body, the magnetic state of the body will be switched to a B state or the 0 state as indicated by the point 29 on the loop, and, when the driving force is relieved, the body will retain a residual magnetism indicated by the point 22 on the B axis.

If, while the body represented by the loop has a resdual magnetism of value indicated by the point 16, a half select 0 pulse as indicated at 23 is applied thereto and then relieved, the degree of magnetism thereafter remaining in the body may, for example, be indicated by the point 26. Similarly, if when the body is at a magnetic state indicated by the point 22, a half select 1 pulse 24 is applied thereto and relieved, the magnetic state of the body remaining thereafter may, for example, be indicated by the point 3%. It should be noted that the actual points 26 and 3% shown on the diagram are exaggerated displacements which are selected for the purpose of clarity and are not intended to be indicative dimensionally of any exact condition prevailing for any given body.

When the body is at a magnetic state as indicated by the point 16, the application of a full select 0 pulse 18 will produce an output voltage indicated by the dimension V If the same full select pulse is applied when the body had a residual state as indicated by the point 26, :a lesser output voltage will be generated. This voltage is indicated at ,V Similarly, if the magnetic body had a residual state as indicated by the point 22 and a full select 0 pulse 18 were applied, an output voltage ,v, would be generated, and if the magnetic state had been at the point indicated at 30, upon an application of a full select 0 pulse, greater output would have occurred as indicated at V It will be evident that the degree of squareness is indicated by the displacement between points 22 and 30, and by the displacement between points 26 and 16.

Theratio V V provides a highly satisfactory measure of squareness in that V is a relatively absolute value of disturbance resulting from lack of perfect squareness and ,.V accommodates for the fact that various materials will have hysteresis loops of various BH ratios. Thus, for a high value of B, a greater displacement between points 22 and 30 may be tolerated than for a low value of B. Accordingly, hereinafter, :squareness ratio will be merely referred to as the expression V V and the following discussion will consider only values of V and V in the considerations of this squareness ratio.

The usual techniques employed in the production of ferrospinel bodies involve the mixing of commercially pure fine particles of oxides of desired materials in desired proportions. Such mixing is accomplished, for example, by wet ball milling to form a slurry. The slurry is thereafter dried and the resulting dry cake is ground to a line powder. This powder is then placed in a suitable container and calcined in air at temperatures of approximately 600 C. to 1000 C. for time intervals ranging from 3.0 minutes to 180 minutes. The actual temperatures and times employed vary with the compositions involved.

After calcining, the material is again milled and there is added to. the material suitable binder and lubricant materials to facilitate the subsequent molding operation. The binder may be polyvinyl alcohol added in the amount of approximately 3% by weight and the lubricant may be a dibutyl phthalate added in the amount of approximately 1% by weight.

The resulting mixture is then molded into the for-m of a desired body which may be of toroidal or of other desired shape. The body in this condition is termed a green body.

After the molding operation the green body may be heated to approximately 600 C. and the binder and lubricant which are organic compounds, are driven therefrom.

After the binder and lubricant are driven off, the green molding is placed in a furnace and sintered at temperatures ranging from approximately 110i) C. to 1500 C. for time intervals ranging from approximately to 30 minutes depending upon its composition and the characteristics desired. After the sintering step, the sintered body is removed from the furnace and either left to cool in air or, in some instances, furnace cooled to an intermediate temperature of approximately 960 C. and then cooled in air to room temperature.

The foregoing process steps of mixing, calcining, adding binders and lubricants, molding and sintering are well known in the art. These process steps or variables are adjusted to produce bodies having specific values of coercivity and other desired characteristics. The novel process operation disclosed herein is diagrammed in FIGURE 2 and, as previously noted, involves interrupting the sintering cycle by cooling the cores to temperatures approaching room temperature.

FIGURE 2 shows a plot of sintering temperature versus timein which t indicates the time required for cores introduced into a furnace to come up to furnace temperature. The actual sintering temperature employed will be determined by the core composition involved and the characteristics desired in the finished product. however, in any event, after the cores have been positioned in the furnace for a time interval 13, they will have arrived at furnace temperature and this temperature condition will prevail for a time interval 1 In the carrying out of the invention, the time interval t is preferably extended to at least half the total sintering time, however, under some conditions of composition and desired final characteristics, this limitation is not critical. The essential consideration is that the cores remain in the furnace sufficiently long to insure their having been soaked throughout to the sintering temperature.

After time the cores are removed from the furnace and cooled in air. Generally, the cores are handled in small flat containers in which the cores may be spread over the surface of the container. to permit rapid and uniform heating and cooling of the core and to permit access by the atmosphere to the core surfaces. Thus, if this container is withdrawn from the furnace and placed on a metal plate at room temperature, a quenching effect will occur and the temperature of the cores will drop to the quench temperature along a curve as is indicated during time i in FIGURE 2.

After the cores have reached the quench temperature, they will be retained at this temperature for a time interval 1 the duration of this interval is not critical, the essential consideration being that the cores be reasonably uniformly cooled, thus 1 may be of the order of minutes or days.

After the quenching operation, the cores are re-admitted to the furnace wherein, during time interval t they will be reheated to the sintering temperature. Thereafter, the cores are held at sintering temperature for a time interval r The total time of t and I is equal to the optimum time interval for sintering the particular core composition involved to achieve the particular characteristics desired. In other words, the total time the cores are retained at sintering temperature in the present process is substantially identical to the total time which would be employed with an uninterrupted or conventional sintering process.

After time t the cores are removed from the furnace and handled in any conventional manner. Some cores are immediately quenched to room temperature and some cores are first quenched to an intermediate temperature, however, any of the conventional sintering techniques may be employed following the sintering cycle as indicated by time intervals to shown in FIGURE 2.

In the following chart, indicated as Chart 1, there are set forth four compositions, each comprising a manganeseferrite ferrospinel. The compositions set forth and their respective characteristics, as will be hereinafter discussed in connection with FIGURES 3, 4a and 4b, make it evident that advantageous results are obtained by the sintering cycle of this invention over a wide range of compositions forming manganese-ferrite systems.

CHART I [Compositions in 11101 percent] 1W8 CM: NCM K-l07 FIGURE 3 is a chartshowing the relation between the V V ratio and the tempenature to which the cores are cooled in the quenching cycle, i.e., the temperature of the cores during time t; of FIGURE 2.

The data for the charts of FIGURE 3 was obtained from K-107 material processed to have a coercivity H of 1.8 oersteds. The chart shows interrupted firing quench temperatures of 25 C., 100 C., 150 C., 250 C., and 500 C., and also shows a continuous firing, i.e., uninterrupted firing. Plotted against these values are values ,V V and V V The plots are numbered 42, 44, and 46, respectively. From plot 46, it will be evident that a substantial increase in the V V ratio takes place when the interrupted firing quenching temperature is carried downwardly below 250 C. and maximum benefit is obtained when the quenching is carried to 100 C. or below. While it is to be expected that the maximum temperature at which the interrupted firing quench may be accomplished will vary somewhat for various compositions and also vary somewhat in view of possible variations of other of the process variables, it will also be evident that these variations will be limited to a reasonable range approaching room temperature and well below the lower calcining temperatures of ap proximately 600 C.

In FIGURE 40, there are shown in four columns indicated generally at 62, 64, 65, and 68, short lines indicating the relationships between various core characteristics obtained by continuous firing and interrupted firing.

The columns are headed M8, K-107, NCM, and CM, respectively, indicating the compositions of the respective materials and all of the materials in FIGURE 4a were processed to produce cores having approximately 1.1 oersteds coercivity.

In FIGURE 4a, the lines indicated V ,V and V connect data points and indicate the changes in these characteristics between continuously fired and interrupted fired cores for each of the compositions shown. The lines V V indicate the changes in squareness ratio resulting from the improved firing process. It will be noted that in each example, the squareness ratio improves substantially.

Also shown in FIGURE 4a, are lines indicated T connecting switching time data points for cores produced by two firing techniques. Switching time T is measured in microseconds and represents the time between the rise and fall of the core response to a full select drive current measured at a predetermined milli-volt level approximately equal to of the voltage output of V It will be observed that in each instance, the improved firing produces either substantially the same or an improved switching speed.

FIGURE 4b contains three columns of data indicated generally at 72, 74 and 76 for materials K107, NCM and CM, respectively when processed to produce cores of 1.5 oersteds coercivity.

FIGURE 4 shows V and V data points and resulting V l V ratio data points for each of the materials listed. It will be observed that in each of these examples the interrupted firing technique provides an improved squareness.

At these coercivities, the switching times T decrease slightly for K-107 and NCM materials and remain substantially the same for CM materials. This change in switching speed is relatively minor and is thus considered not particularly disadvantageous.

From the foregoing data, it will be evident that very definite improvement in squareness ratio of cores is provided by the split firing technique disclosed and claimed. It should also be noted that ferrospinel materials exhibiting square hysteresis loop characteristics are of the manganese-ferrite system (Magnetic and Electrical Properties of the Binary Systems MO.Fe O by J. L. Snoek, Physica III, No. 6, June 1936, page 463). To this system is added, oxides of various bivalent metals in order to modify the properties of the basic ferrospinel system. The foregoing data is sufficiently broad to show that the interrupted firing technique disclosed improves the squareness of the ferrospinel system regardless of the addition of numerous additives and regardless of variations of the process variables employed to vary the coercivity of the resulting body.

While the invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. The process of producing rectangular hysteresis loop ferrite structures of the manganese ferrite system having enhanced rectangularity of the hysteresis cl1aracteristic including the steps of:

forming a mixture comprising 38 to 44.4 mol percent Fe O 51.1 to 60 mol percent MnO and up to 5 mol percent of an additive selected from the group consisting of CuO, CrO, and NiO;

calcining said mixture at a temperature in a range between 600 to 1000 C.;

finely dividing said calcined mixture;

molding said finely divided mixture into a body of a predetermined density; and,

sintering said molded body, said sintering comprising the sequential steps of heating said molded body to a temperature in a range between 1100 to 1500 C. for a first sintering period of about 15 to 30 minutes, cooling said heated body in air to a temperature below 250 C. and reheating said cooled body to a temperature in said range for a second sintering period, Where said second sintering period is at least equal to one half the total sintering time.

2. The process of producing rectangular hysteresis loop ferrite structures of the manganese ferrite system having enhanced rectangularity of the hysteresis characteristic including the steps of:

forming a mixture comprising 38 to 44.4 mol percent Fe O 51.1 to 60 mol percent MnO and up to 5 mol percent of an additive selected from the group consisting of CuO, CIO and NiO;

calcining said mixture at a temperature in a range between 600 to 1000 C.;

finely dividing said calcined mixture;

molding said finely divided mixture into a body of a predetermined density; and,

sintering said molded body, said sintering comprising the sequential steps of heating said molded body to a temperature in a range between 1100 to 1500 C. for a first sintering period of about 15 to 30 minutes, cooling said heated body in air to a tempera ture below C. and reheating said cooled body to a temperature in said range for a second sintering period, where said first sintering period is at least equal to one half the total sintering time.

References Cited in the file of this patent UNITED STATES PATENTS 2,818,387 Beck et al. Dec. 3 1, 1957 2,905,641 Esveldt et a1. Sept. 22, 1959 2,988,508 Geldermans et a1. June 13, 1961 FOREIGN PATENTS 167,499 Australia Apr. 18, 1956 201,673 Australia May 2, 1956 204,795 Austria Aug. 10, 1959 532,384 Belgium Apr. 7, 1955 1,125,577 France July 16, 1956 67,809 France Oct. 14, 1957 (Addition to No. 1,121,088)

797,168 Great Britain June 25, 1958 OTHER REFERENCES Harvey et al.: RCA Review, September 1950, pages 344-349. 

1. THE PROCESS OF PRODUCING RECTANGULAR HYSTERESIS LOOP FERRITE STRUCTURES OF THE MANGANESE FERRITE SYSTEM HAVING ENHANCING RECTANGULARITY OF THE HYSTERSIS CHARACTERISTIC INCLUDING THE STEPS OF: FORMING A MIXTURE COMPRISING 38 TO 44.4 MOL PERCENT FE2O3, 51.1 TO 60 MOL PERCENT MNO AND UP TO 5 MOL PERCENT OF AN ADDITIVE SELECTED FROM THE GROUP CONSISTING OF CUO, CRO, AND NIO; CALCINING SAID MIXTURE AT A TEMPERATURE IN A RANGE BETWEEN 600 TO 1000*C.; FINELY DIVIDING SAID CALCINED MIXTURE; MOLDING SAID FINELY DIVIDED MIXTURE INTO A BODY OF A PREDETERMINED DENSITY; AND, SINTERING SAID MOLDED BODY, SAID SINTERING COMPRISING THE SEQUENTIAL STEPS OF HEATING SAID MOLDED BODY TO A TEMPERATURE IN A RANGE BETWEEN 1100 TO 1500* C. FOR A FIRST SINTERING PERIOD OF ABOUT 15 TO 30 MINUTES, COOLING SAID HEATED BODY IN AIR TO A TEMPERATURE BELOW 250*C. AND REHEATING SAID COOLED BODY TO A TEMPERATURE IN SAID RANGE FOR A SECOND SINTERING PERIOD, WHERE SAID SECOND SINTERING PERIOD IS AT LEAST EQUAL TO ONE HALF THE TOTAL SINTERING TIME. 